PS plates containing a positive type photosensitizing agent comprising a diazo compound and a phenolic resin as main components, or containing a negative type photosensitizing agent comprising an acrylic monomer or pre-polymer as main components, are in use today as lithographic offset printing plate precursors. However, these plate precursors have a low sensitivity, and in all cases, plate making is carried out using a contact exposure with a film original on which the image has been previously recorded. On the other hand, as a result of the progress which has been made with computer image processing and high capacity data storage and data communication techniques, computers have been used in recent years for all stages from original input through correction, and editing, layout and paging. The information is then forwarded to remote terminal plotters in real time, using high speed communication networks or satellite communications, to put such systems into practical operation. Electronic editing systems are required in particular in the newspaper printing field where real time characteristics are essential. Furthermore, even in those fields where the original is stored in the form of a film original and a plurality of printing plates are made on the basis of this original, it is thought that the original may be digitally stored as a result of the development of ultra-high capacity recording media such as optical disks, for example.
However, direct type printing plate precursors wherein a printing plate is made directly from the output of a terminal plotter are still impractical. Even when an electronic compilation system has been used, the output is transferred to a silver salt photographic film. The printing plates are then made indirectly using these films, by contact exposure on PS plates. One reason for employing a contact exposure is that it has proved difficult to develop direct type printing plate precursors having a high enough sensitivity to enable a printing plate to be made within a practical period of time using the light source of an output plotter (for example, a He--Ne laser or a semiconductor laser).
Electrophotographic photoreceptors are considered to be photosensitive bodies having a sufficiently high photosensitivity for use in providing direct type printing plates. Many electrophotographic printing plate precursors wherein a toner image is formed and the photoconductive layer is subsequently removed from the non-image parts are known in the art. For example, electrophotographic printing plate precursors have been disclosed, for example, in JP-B-37-17162, JP-B-38-6961, JP-B-38-7758, JP-B-41-2426, JP-B-46-39405, JP-A-50-19509, JP-A-50-19510, JP-A-52-2437, JP-A-54-145538, JP-A-54-134632, JP-A-55-105254, JP-A-55-153948, JP-A-55-161250, JP-A-56-107246, JP-A-147656 and JP-A-57-161863. (The terms "JP-A" and "JP-B" as used herein signify an "unexamined published Japanese patent application" and an "examined Japanese patent publication", respectively.)
Electrophotographic printing plate precursors containing an oxazole compound as an organic photoconductive compound in an aqueous alkaline or alcoholic solution soluble binder resin, and a photoconductive layer obtained by adding a sensitizing dye to this mixture and then coating onto an aluminum plate have been disclosed in JP-A-56-107246. Furthermore, electrophotographic printing plate precursors which contain oxadiazole compounds as organic photoconductive compounds, poly-condensed ring quinone pigments as charge generating agents for these compounds, and alkali soluble carboxyl group containing polymers have been disclosed in JP-A-56-146145. Moreover, electrophotographic printing plate precursors which contain hydrazone compounds as organic photoconductive compounds and which contain pigments such as phthalocyanine pigments or quinacridone pigments, or dyes, as charge generating agents for these compounds and binder resins which are soluble in aqueous or alcoholic solvents have been disclosed in JP-A-62-54266.
It is usually necessary to remove the non-image parts with an alkaline etching fluid and to expose a hydrophilic surface when electrophotographic photoreceptors of this type are used as printing plates. Thus, a binder resin which is soluble in alkaline solvents or which is swelled and removed by alkaline solvents is often used as the binder resin. However, in comparison to the polycarbonate resins, for example, which are widely used as binder resins for electrophotographic photoreceptors, the resins which are dissolved in, or swelled by, alkaline solvents generally have poor compatibility with most organic photoconductive compounds such as the oxazoles, hydrazones, oxadiazoles and pyrazolines, for example. Consequently, where the above-described compounds are dissolved and included in a printing plate, the organic photoconductive compounds tend to separate and precipitate upon storage. Furthermore, these organic photoconductive compounds have poor solubility in the etching fluid such that the washing out or etching properties of the non-image parts with the etching fluid is inadequate. A further disadvantage is that base staining occurs in the white base parts when printing is carried out.
Moreover, as shown in the Examples described hereinafter, electrophotographic printing plate precursors which contain organic photoconductive compounds generally have poor charging properties and charge retention properties in the dark. Consequently, the potential difference between the image portions and the non-image portions is reduced such that toner development may become inadequate, and the available time between charging and development is limited. As a result, such electrophotographic printing plate precursors are of little practical value. Thus, there are various problems associated with electrophotographic printing plate precursors which contain organic photoconductive compounds dissolved in a binder resin.
Moreover, there are also electrophotographic printing plate precursors in which the organic photoconductive compound is not dissolved in the binder resin as described above. This type of electrophotographic printing plate precursor has a photoconductive layer in which an organic photoconductive pigment such as a phthalocyanine pigment is dispersed in an aqueous alkaline or alcoholic solution soluble binder resin. For example, electrophotographic printing plate precursors having a photoconductive layer comprising a phthalocyanine pigment dispersed in a phenolic resin and provided on an aluminum plate have been disclosed in JP-A-55-105254 and JP-A-55-161250, but the sensitivity of these plate precursors is inadequate. Electrophotographic photoreceptors of this type wherein a phthalocyanine pigment is dispersed in a binder resin and not containing organic photoconductive compounds such as hydrazone compounds and oxazole compounds, exhibit an induction effect which reduces the sensitivity, as described in Current Problems in Electrophotography, Weigl, p. 278, Walter de Gruyter (1972). Furthermore, it is also known that this induction effect can be reduced and the sensitivity can be increased by adding electron attracting compounds such as tetranitrofluorenones and trinitrofluorenones, for example, to the photoreceptor, as described in Denshi Shasshin Gakkaishi 60, 116, 20 (1982). However, these electron attracting compounds are toxic and are difficult to use in practice.
The development of electrophotographic printing plate precursors having a high sensitivity, good storage properties, and good etching properties, has been desired for overcoming the above-described problems.